Today, modern heating boilers are fired with heating fuel oil or natural gas. When burning the hydrocarbons forming these fuels, flue gases are generated which contain certain undesired components resulting in detrimental contamination of the atmospheric air. The nitrogen oxides or nitric oxides (NO.sub.x) are an especially detrimental component. A not unessential part of these NO.sub.x emissions is generated by the flue gases of heating boilers fired with hydrocarbons.
NO.sub.x is detrimental since it contributes to the generation of the photochemical smog in a complicated manner. By this, the ozone concentration near to the ground can be increased. Furthermore, NO.sub.x results in an overacidification of the ground and of water. Moreover, it represents a health risk especially for asthmatics.